The concept of directing the passage of liquid through a heater exchanger by making its path spiral instead of linear is shown in U.S. Pat. No. 3,020,026 to Peeps et al, in U.S. Pat. No. 3,446,032 to Bottum and in U.S. Pat. No. 3,545,063 to Mitchell, all of which show straight tube exchangers together with helical deflectors, instead of helical volute tube exchangers where the volute member snugly fits both a core member and a surrounding shell member and includes finned peripheries.
The establishment of spiral paths is often accomplished by helical wire members as shown in the preceding patents, but may be done by helical surface concentric tubes such as in U.S. Pat. No. 4,086,958 to Habdas wherein a helical finned inner tube lies within a helically grooved outer tube, although the pitches of the two members are very different, and the outer member provides a large cross section helical flow path which is entirely unoccupied by fins and unobstructed. Other examples of multiple different pitch helical members are shown in U.S. Pat. Nos. 556,630 to Hoberecht; 1,893,484 to Belt and 3,335,790 to Aranyi et al.
U.S. Pat. No. 2,146,141 to Harris shows a helical volute, which is in a straight walled outer shell and recessed in the helical groove of a core member. However, this structure does not include fins on the volute, and if fins were added, the function of the helical groove would be lost, since the fins would tend to stop all flow in each groove.